The present disclosure relates to an interference cancellation system (ICS) repeater that may be used in a wireless network, and more particularly to, an interference cancellation device and method of an ICS repeater using a leveler that effectively cancel only interference signals by using a leveler in an ICS repeater and enable an ICS repeater design for an LTE wireless network that needs a short system delay.
In general, an interference cancellation system (ICS) refers to a system for solving oscillation issues due to feedback signals between transmission and reception antennas in a wireless repeater, and more particularly to, a wireless repeater employing features that apply interfere cancellation to a typical wireless repeater to predict a feedback signal and subtract the predicted signal from an input original input to cancel the feedback signal.
FIG. 1 is a schematic diagram of an adaptive ICS repeater that uses an interference cancellation method using a general adaptive algorithm, and an interference cancellation engine 40 is disposed between transmission and reception antennas 10 and 30 and includes a delay unit 41, a coefficient generator 42, and a finite impulse response (FIR) filter 43.
The original signal d(n)=s(n)+y(n) received through the reception antenna 10 passes through a subtractor 21 and outputs an error signal e(n) devoid of an interference signal.
In this case, the coefficient generator 42 of the interference cancellation engine 40 calculates the correlation between the error signal e(n) devoid of the interference signal generated through the subtractor 21 and a reference signal x(n) delayed for a certain time through the delay unit 41 to calculate a prediction coefficient w(n).
The interference signal prediction coefficient w(n) generates a in-phase predicted feedback signal y′(n) by convolution with the reference signal x(n) through the FIR filter 43 and the predicted feedback signal y′(n) output, to the transmission antenna 30 side, a signal devoid of the interference signal by subtraction from the original signal d(n) through the subtractor 21.
The adaptive ICS repeater that uses interference cancellation using such an adaptive algorithm generally uses a least mean square (LMS) or recursive least square (RLS) adaptive algorithm, which has a characteristic in that interference cancellation capability varies depending on the characteristic of an input signal source because such an adaptive algorithm uses the correlation between the error signal e(n) and the reference signal x(n) to predict the feedback signal and generate an in-phase feedback signal.
In particular, in a case where a narrow-band signal is input such as a subcarrier portion of a long term evolution (LTE) or multi-carrier modulation, orthogonal frequency division multiplexing (OFDM) signal, there are limitations in that the interference signal may not effectively be cancelled by the magnetic correlation characteristic of the input signal, and rather unnecessary noise signals are generated.
In order to solve these limitations, unnecessary noise signals in a signal-free band are typically cancelled by using a FA (carrier) filter or not recognized as the narrow-band signal by a channel-type design.
However, since such an FA filter or channel type is a digital filter and significantly affects a system delay, there is a limitation in that it is difficult to design a short delay (4.6875 μs) in an LTE network.
The reason why a short delay design is required in the LTE network is prevent interference between channels and a ‘Cyclic Prefix’ is inserted as a signal inserted into an interval section. However, if there is no signal in a guard interval section, the orthogonality of a subcarrier is broken and interference between channels occurs. In order to prevent it, a portion of a signal in the rear part of a symbol section is copied and inserted and it is referred to as ‘Cyclic Prefix’.
In the LTE network, the length of the cyclic prefix is 4.6875 μs and in the case of a wireless repeater, interference between channels does not occur only when a system delay is short than or equal to 4.6875 μs.
FIG. 2 is a schematic diagram of another ICS repeater to which a magnetic correlation cancellation device according to the related art is applied, and which is disclosed in Korean Patent Publication No. 10-2013-0054305 (Interference cancellation method of ICS repeater).
The related art is to cancel magnetic correlation to solve a limitation in inputting a narrow-band signal, the method of cancelling the magnetic correlation cancels narrow-band signals among feedback signals and leaves only wide-band white noise components to use the white noise component as the reference signal x(n), and since the narrow-band signal is cancelled from the reference signal x(n) and only the white noise remains, a level of a signal is weak and so there is a limitation in that an adaptive speed at which the prediction coefficient w(n) is calculated gets slow.